An optical connector assembly is an essential device required for constituting an optical communication system, an optical measuring system or the like system wherein various instruments, apparatuses, equipments or optical circuit components are connected to each other via optical fibers. A main function of the optical connector assembly is to connect an optical fiber to an optical element (in the form of a light emitting element or a light receiving element) and moreover connect the optical fiber to an electrical circuit or electrical circuits.
FIGS. 1 and 2 typically show a conventional optical connector assembly by way of a perspective view, respectively.
The optical connector assembly as shown in FIG. 1 comprises an optical connector X.sub.1 having end parts A.sub.1 and A.sub.2 of a two-core type optical fiber A protruded therefrom to firmly hold the optical fiber A and a photoelectric converting module X.sub.2 including a photoelectric converting element. With this conventional optical connector assembly, connecting of the optical connector X.sub.1 to the photoelectric converting module X.sub.2 is carried out by fitting the optical connector X.sub.1 to the photoelectric converting module X.sub.2 and then actuating a locking mechanism P.sub.2. In addition, electrical/mechanical connection of the photoelectric converting module X.sub.2 to a printed-circuit board G is carried out by using a lead frame P.sub.1 which is arranged below the photoelectric converting module X.sub.2.
On the other hand, the conventional optical connector assembly as shown in FIG. 2 includes an integrated type optical module X.sub.3 in which a photoelectric converting element is received so that the foremost end of an optical fiber A comes in contact with a light emitting surface or a light receiving surface of the photoelectric converting element. An electric plug P.sub.2 is attached to the end surface of the optical module X.sub.3 so that the optical module X.sub.3 is coupled to an electrical module X.sub.4 by fitting the electrical plug P.sub.2 into an electrical jack (not shown). Connection of the electrical module X.sub.4 to a printed-circuit board G is carried out by using a lead frame P.sub.1 in the same manner as described above with reference to FIG. 1.
In this manner, connecting/disconnecting of the conventional optical connector assembly is accomplished by a simple fitting operation for the aforementioned respective components.
Thus, the conventional optical connector assembly can be used without any particular problem for an apparatus or an equipment installed in a calm and dustless environment, e.g., an audio apparatus, an apparatus for hospital, office or the like facilities. However, when the conventional optical connector assembly is used for an apparatus or an equipment operable in a severe environment including vibration, noisy sound, various contaminated material, moisture or dust, e.g., facilities in a factory, an industrial machine or the like, there arise problems that electrical noise is generated due to vibration, noisy sound or the like, a connector falls down naturally, and a running life of a connector and associated components is shortened due to the presence of oily contaminated material, dust or the like foreign material.
In addition, since no seal is employed for connecting/ disconnecting locations in components constituting the conventional optical connector assembly, there arises another problem that no optical communication can be made because oil, dust or the like foreign material enters the interior of the optical connector assembly via connecting/ disconnecting locations. For the reason, requirements have been raised for developing an optical link which can be used in field under a severe environmental condition due to invasion of water, oil, dust or the like foreign material, even when optical communication is used between controllers for machines (robots, machine tools, presses etc.) installed for factory automation.
Hitherto, the conventional optical connector assembly is commercially sold in a stationary state wherein an optical fiber A is immovably held in a module X.sub.1 or X.sub.3 as in the case of the optical connector X.sub.1 shown in FIG. 1 or the integrated type optical module X.sub.3 as shown in FIG. 2. Thus, there arises another problem that it is difficult to adequately adjust a length of the optical fiber A in field by a cutting operation or a connecting operation, because connection or disconnection of the optical fiber A in field is substantially impossible.
When an optical fiber is fitted into an optical connector, hitherto, the optical fiber A is first cut to a predetermined length by actuating a plier, a nipper or the like tool. Thereafter, to make the rugged end surface of the optical fiber A flat correctly, a sheath is removed from the optical fiber by a distance of 1 to 2 mm as measured from the end surface of the optical fiber A and then the end surface of a core A.sub.a of the optical fiber A is brought in contact with a hot plate 101 or an abrasive paper (not shown), as shown in FIG. 3. After heating or grinding the end surface of the core A.sub.a, a rubber seal is fitted round the core A.sub.a at a suitable position away from the end surface of the core A.sub.a.
However, if the optical fiber A is excessively heated or ground, there is a danger that the sheath may be deformed, as shown in FIG. 4, resulting in the optical fiber A being undesirably damaged or injured. In such a case as described above, it becomes impossible to insert the optical fiber into a hole on the optical connector. Even though the optical fiber could be inserted into the hole, an optical coupling efficiency of the optical fiber and other optical factors may be degraded.
In a case where the conventional optical connector assembly is used as an optical signal sending module for optical communication, light generated by a light emitting diode (hereinafter referred to as a LED) is transmitted via an optical fiber but the light is dampened more and more as a length of the optical fiber, i.e., a distance of transmission of optical signals is elongated.
To assure that a constant quantity of light is always received regardless of how far a distance of signal transmission is elongated, a circuit structure as shown in FIG. 5 has been heretofore employed.
Referring to FIG. 5, an optical signal sending module 102 includes a LED 103 and a switching transistor 104 as essential component for the purpose of optical communication. In response to inputting of signal data SIG via a terminal T.sub.2, the transistor 104 is turned on or off and thereby the LED 103 is turned on or off. The module 102 is connected to a power supply source circuit 105 via a terminal T.sub.1. The circuit 105 includes a direct current power supply source 106 and a plurality of fixed resistors R.sub.1, R.sub.2, --- R.sub.n connected to the power supply source 106 in parallel therewith. One of a plurality of terminals S.sub.1, S.sub.2, --- S.sub.n on the circuit 105 side is connected to the terminal T.sub.1 on the optical module 102 side so that an intensity of light generated by the LED 103 is correctly adjusted by feeding an adequate intensity of electric current to the LED 103 corresponding to the present distance of signal transmission.
A structure wherein a variable resistor 107 is connected to the LED 103 in the optical module 102 in series as shown in FIG. 6 is known as other example of the foregoing kind of prior art. Additionally, arrangement of an adjustable resistor 108 outside of the optical module 102 as shown in FIG. 7 is known as another example of the prior art. With the conventional structure as shown in FIGS. 6 and 7, however, it is necessary that a suitable resistor on the circuit 105 side is selected from among the plural resistors for the optical module 102 and the thus selected resistor is connected to the optical module 102 at a position outside of the optical module 102. Particularly, with the conventional structure as shown in FIG. 6, it is necessary that the variable resistor 108 is adjusted correctly. For the reason, there arises a problem that handling of the optical fiber in field and a setting operation for a length of the optical fiber in field become complicated.
In a case where the aforementioned conventional optical connector assembly is used as a signal receiving module for optical communication, in response to receiving of an optical signal, an output from the optical connector assembly is delivered to a printed-circuit board electrically connected to the optical connector assembly, whereby the signal in the form of an output from the optical connector assembly is processed in a signal processing circuit mounted on the printed-circuit board. However, it is essential, from viewpoint of a necessity for demodulating the optical signal thereby to check the content of data included in the optical signal, that an optical signal receiving circuit for converting the optical signal into an electrical signal thereby to discriminate the content of a logical level of the electrical signal is arranged in each signal receiving section in the optical communication system.
In fact, the applicant of the present invention invented an optical signal receiving circuit employable for an optical communication system wherein the content of a logical level of an input signal can be discriminated without any adverse effect of the input signal on an offset voltage and moreover a duty ratio of the discriminated output signal can be maintained normally and he filed an application for patent later (refer to Japanese Patent Application NO. 175694/1987).
This prior invention discloses a comparator circuit as shown in FIG. 8 which is constructed in a two-stage structure comprising a first comparator 110 and a second comparator 120 situated at the later stage. The conventional comparator circuit is constructed such that the content of a logical level of a received signal (input signal) is discriminated independently of an offset voltage of the received signal by making comparison in the first comparator 110 as to a level of the received signal and incorrect variation of a duty ratio of the discriminated signal is automatically corrected by making comparison in the second comparator 120 as to a level of the discriminated signal to reverse a manner of inputting a signal corresponding to the input signal and a voltage (signal) corresponding to the threshold voltage relative to the first comparator 110.
With the above construction, the comparator circuit can accomplish the initially intended object without fail. As shown in FIG. 8, however, the first comparator 110 and the second comparator 120 are activated with a constant magnitude of voltage V.sub.cc derived from the power supply source.
Specifically, when a transistor 111 in the first comparator 110 at the first stage fails to be activated because a voltage appearing at a collector of the transistor 11 is held at a level of V.sub.cc, i.e., when an output derived from the comparison made in the first comparator 110 is held at a logical "1" level, a value of voltage indicative of the output derived from the comparison is raised up near to the voltage V.sub.cc appearing at the collector of the transistor 11.
On the other hand, the second comparator 120 is likewise activated with the voltage V.sub.cc derived from the power supply source.
In this case, an input voltage required for normally operating the second comparator 120 is set to a voltage value substantially equal to about 80% of the voltage V.sub.cc of the power supply source depending on characteristics of the second comparator 120.
Therefore, when an output (a signal remaining at a logical "1" level) derived from the comparison in the first comparator 110 the output being boosted near to the voltage V.sub.cc of the power supply source, is inputted into an input terminal of the second comparator 120, there arises a problem that the second comparator 120 may incorrectly be operated, resulting in an exact output failing to be obtained.
FIG. 9(c) shows a time chart which illustrates a desirable output wave form of the comparator 120. Once the second comparator 120 is incorrectly operated, phase deviation occurs, as shown in FIG. 9(a). Otherwise, a signal may be deformed at the time of signal rising, as shown in FIG. 9(b).